1. Field of the Invention
The disclosed technology generally relates to methods of forming patterns of differently doped regions at a surface of a semiconductor substrate, and more particularly relates to such methods that can be used in fabrication processes for photovoltaic cells.
2. Description of the Related Technology
In methods for fabricating advanced photovoltaic cells, there can be a need for providing patterns comprising differently doped regions such as, for example, a local back surface field (BSF), a selective emitter, or emitter regions and BSF regions. Such differently doped regions can be formed in, for example, an interdigitated back contact (IBC) cell. Formation of such doped regions can leads to increased complexity of and increased cost of photovoltaic cell fabrication processes.
Patterned doped regions can, for example, be formed in a semiconductor substrate by a dopant diffusion process, e.g., after having provided a mask (e.g., a deposition mask, or a growth mask). Formation of the patterned doped regions by a diffusion process can lead to one of several undesirable results. For example, diffusion processes can lead to the presence of a doped glass that needs to be removed after diffusion. Furthermore, diffusion processes can lead to a high surface dopant concentration, which may be disadvantageous when the diffusion process is used e.g. for forming an emitter, because the high surface dopant concentration can give rise to a high surface recombination velocity resulting in a low open-circuit voltage. In addition, diffusion processes can give rise to poor blue response, resulting in poor short-circuit current density. Furthermore, diffusion processes can lead to a doping profile with a thickness (depth) below about 1 micrometer, which may be disadvantageous when the diffusion process is used e.g., for forming an emitter and is combined with a silicide-based contact metallization. In order to reduce the high surface dopant concentration and/or increase the depth of the doping profile after such diffusion processes, additional drive-in steps may be performed, which can lead to an increase the overall number of process steps and thus lead to an overall increase in the complexity of the solar-cell fabrication processing.
Alternatively, patterned doped regions can, for example, be formed by ion implantation. The main drawback of this approach is the crystal damage introduced by the implantation process. This damage can be removed by performing a high-temperature annealing process that is also used to activate the implanted dopants.